B&B Electronics GW-7472 - Manual - Manual (Page 55)

GW-7472 User Manual, Version 1.0, Dec 2010 --- 55
Appendix B: Glossary
1. ARP (Address Resolution Protocol)
Consider two machines A and B that share a physical network. Each has an assigned IP
address IP
A
and IP
B
, and a MAC address the MAC
A
and MAC
B
. The goal is to devise low-
level software that hides MAC addresses and allows higher-level programs to work only
with the IP addresses. Ultimately, however, communication must be carried out by the
physical networks using whatever MAC address scheme the hardware supplies.
Suppose machine A wants to send a packet to machine B across a physical network to
which they are both attached, but A only has the Internet address for B, IP
B
. The question
arises: how does A map that address to the MAC address for B, MAC
B
?
ARP provides a method of dynamically mapping 32-bit IP address to the corresponding 48-
bit MAC address. The term dynamic is used since it happens automatically and is normally
not a concern for either the application user or the system administrator.
2. Clients and Servers
The client-server paradigm uses the direction of initiation to categorize whether a program
is a client or server. In general, an application program that initiates peer to peer
communication is called a client. End users usually invoke client programs when they use
network services.
Most client programs consist of conventional application program develop tools. Each time
a client program is executed; it contacts a server, sends a request and waits for a response.
When the response arrives, the client program continues processing. Client programs are
often easier to develop than servers, and usually require no special system privileges to
operate.
By comparison, a server is any program that waits for incoming requests from a client
program. The server receives a request from a client, performs the necessary computation
and returns the result to the client.